1. Field of the Invention
The present invention relates generally to the art of optical inspection of semiconductor wafers, and more specifically to a high throughput brightfield and darkfield wafer inspection system having image processing redirected from the mechanical and electronics segments of the inspection system to the optical domain.
2. Description of the Related Art
Semiconductor wafer inspection techniques have historically utilized brightfield illumination, darkfield illumination, or spatial filtering. Brightfield imaging is not generally sensitive to small particles. Brightfield imaging tends to scatter small particles away from the collecting aperture, thereby resulting in reduced returned energy. When a particle is small compared to the optical point spread function of the lens and small compared to the digitizing pixel, the brightfield energy from the immediate areas surrounding the particle typically contribute a large amount of energy. The small reduction in returned energy resulting from the small particle makes the particle difficult to detect. Further, the small reduction in energy from a small particles often masked out by reflectivity variations from the bright surrounding background such that small particles cannot be detected without numerous false detections. Additionally, if the small particle is on an area of low reflectivity, which may occur for some process layers on wafers and always for reticles, photomasks, and flat panel displays, the resultant background return is low and any further reduction due to the presence of a particle becomes very difficult to detect.
Newer systems utilize broadband brightfield imaging as opposed to traditional monochromatic or narrow band brightfield imaging. Broadband brightfield imaging minimizes contrast variations and coherent noise present in narrow band brightfield systems, but are not sensitive to small particles.
Darkfield imaging is employed to detect small particles on wafers, reticles, photomasks, flat panels, and other specimens. The advantage of darkfield imaging is that flat specular areas scatter very little light back toward the detector, resulting in a dark image. Darkfield illumination provides a larger pixel-to-defect ratio, permitting faster inspections for a given defect size and pixel rate. Darkfield imaging also permits fourier filtering to enhance signal to noise ratios.
Any surface features or objects protruding above the surface of the object scatter more light toward the detector in darkfield imaging. Darkfield imaging thus produces a dark image except where circuit features, particles, or other irregularities exist. Particles or irregularities are generally assumed to scatter more light than circuit features. However, while this assumption permits a thorough inspection for particles on blank and unpatterned specimens, in the presence of circuit features a darkfield particle inspection system can only detect large particles which protrude above the circuit features. The resulting detection sensitivity is not satisfactory for advanced VLSI circuit production.
While some attempts to improve darkfield performance have been attempted, such systems tend to have drawbacks, including drawbacks resulting from the very nature of darkfield illumination. For example, while brightfield illumination floods the entire field of view with light, darkfield illumination is confined to a narrow strip of light. Due to the nature of lasers, the application of light in darkfield illumination tends to be non-uniform and limits the amount of data which can be collected in a particular time period.
Some imaging systems currently available attempt to address problems associated with darkfield imaging. One instrument, manufactured by Hitachi, uses the polarization characteristics of the scattered light to distinguish between particles and normal circuit features, based on the assumption that particles depolarize light more than circuit features during the scattering process. When circuit features become relatively small (less than or on the order of the wavelength of light), the circuit can depolarize the scattered light as much as the particles. As a result, only larger particles can be detected without false detection of small circuit features.
Further, a system employing a combination of a monochromatic darkfield and a monochromatic brightfield imaging for wafer inspection is poorly adapted for inspecting Chemical Mechanical Planarized (CMP) wafers, which often have film thickness variations and a grainy texture. Grainy texture, it should be noted, may also be a result of the metal grain structure of the wafer.
Another attempt to resolve problems associated with darkfield imaging positions the incoming darkfield illuminators such that the scattered light from circuit lines oriented at 0°, 45°, or 90° are minimized. This method is generally effective on circuit lines, but light scattering from corners is still relatively strong. Further, detection sensitivity for areas having dense circuit patterns must be reduced to avoid the false detection of corners.
Prior systems for processing brightfield and darkfield data have relied on different processing techniques. The system of FIG. 1 illustrates a prior system which performed a full processing of a wafer using brightfield imaging followed by darkfield imaging and subsequent processing of the wafer. The problem with this mechanization is that throughput, or the time to process a single wafer, is generally poor, and it does not have the capability to use the combined results from both brightfield and darkfield imaging. As shown in FIG. 1, brightfield imaging 101 is performed on wafer 103, wherein the brightfield imaging has tended to be either monochromatic or narrow band imaging. The wafer image is received via sensor 104, which performs TDI, or Time Delay Integration, and a phase lock loop analog to digital conversion (PLLAD). Data is then directed to input buffer 105, which passes data to defect detector 107. Defect detector 107 uses delay 106 to perform a die-to-die or cell-to-cell comparison of the brightfield image processed wafer 103. The results are then passed to post processor 108 where brightfield defects are determined and passed. The wafer 103 is then illuminated using darkfield illumination 102 in the second run, and all subsequent processes are performed on the darkfield image. The result of this second run is a list of darkfield defects. The typical defect assessment performed by the defect detector and the post processor 108 is to set a threshold above which a feature is considered a defect, and only passing brightfield or darkfield results exceeding such thresholds. This does not completely account for the benefits associated with the combined effects of using brightfield and darkfield, and the amount of time necessary to perform all processing for a single wafer can be significant.
An alternative to the mechanization of FIG. 1 is presented in FIG. 2. The system of FIG. 2 illustrates simultaneous brightfield illumination 201 and darkfield illumination 202 of wafer 203. The simultaneous illumination is typically from a single illumination source, and the system receives the wafer images using dual TDI and PLLAD sensors 204 and 204′. Each sensor 204 and 204′ receives an image of wafer 203 and loads a signal representing that image into input buffer 205 or 205′, such as RAM. From buffer 205 or 205′ the system feeds data to defect detector 207 where data representing the wafer 203 is compared to similar or reference wafer characteristics under the control of delays 206 and 206′. Delays 206 and 206′ each provide timing for a die-to-die or cell-to-cell comparison by defect detector 207. Defect detector 207 uses information from both brightfield and darkfield illumination steps to determine the location of defects on wafer 203. The combined defect list from defect detector 207 is then evaluated using post processor 208 to identify pattern defects and particles.
The drawback in implementing the system illustrated in FIG. 2 is that individual TDI and PLLAD sensors 204 and 204′, input buffers 205 and 205′, and delays 206 and 206′ are highly sophisticated and expensive components, and the use of two of each such components significantly increases the cost of the entire machine. Further, performance of defect detector 207 and post processor 208 requires that all data be available and be evaluated at one time, which can cause significant delays and high processing costs. For example, it is not unusual to see brightfield imaging requiring a very short amount of time while darkfield imaging takes significantly longer. This system also uses monochromatic or narrowband brightfield imaging, which has a tendency to exhibit undesirable contrast variations and coherent noise problems as discussed above.
Spatial filtering is another technique used to enhance the detection of particles. With plane wave illumination, the intensity distribution at the back focal plane of a lens is proportional to the Fourier transform of the object. Further, for a repeating pattern, the Fourier transform consists of an array of light dots. Placement of a filter in the back focal plane of the lens to block out the repeating light dots permits filtering of the repeating circuit pattern and leaves only non-repeating signals, such as particles or other defects. The major limitation of spatial filtering is that only areas having repeating areas or blank areas may be inspected.
There has been little interest in combining brightfield and darkfield techniques due to a lack of understanding of the advantages presented by such a technique. All of the machines currently available employing monochromatic brightfield/darkfield imaging use a single light source for both brightfield and darkfield illumination and do not use a combination of brightfield and darkfield images to determine defects.
Microscopes exist on the market today which combine both monochromatic brightfield and darkfield illumination, and such microscopes have a single light source and provide both illuminations simultaneously, thus making it impossible to separate the brightfield and darkfield results. Such mechanizations simply result in a combined full-sky illumination.
A further limitation of prior systems is that the illumination sources tend to be fixed in place, which also fixes the ability of the system to pick up defects in surfaces or specimens having different physical properties. Typically, a video camera is positioned above the specimen and light is applied to the specimen at a predetermined angle. The application of light to a particle tends to scatter the light, which is then detected by the video camera. If the specimen contains an irregular surface configuration, such as excess material or a semiconductor pattern, the fixed angle of the light source may not optimally scatter the applied light, inhibiting the ability to detect defects. Even for a wafer having a regular semiconductor pattern, orientation of the illumination source provides a different return when a pattern feature is oriented at 0°, 45°, or 90°. Also, the support mechanisms and circuitry associated with the light source tend to be large and bulky, thereby impeding the repositioning capability of the light source.
Another problem with brightfield/darkfield imaging is the use of imaging devices within the same physical space. Components associated with brightfield imaging are generally used for darkfield imaging as well, and several overlapping components exist when using both forms of illumination and detection. However, due to the optical, physical, and other characteristics of components used in brightfield/darkfield imaging, some components tend to provide advantages with one form of illumination and disadvantages for the other illumination scheme. The minimization of the disadvantages associated with a form of imaging improves the ability to detect problems associated with individual specimens.
Another problem associated with wafer inspection systems is the control of light level. Control of light level is particularly complex and critical where a high level of light collection efficiency is desired, and where the gain of the detector is not readily controlled. Prior systems for providing light level control for wafer inspection include providing absorbing glass in the illumination path, and control over the output energy of the laser. These systems either do not perform sufficiently and/or are too costly or complex to use efficiently.
It is therefore an object of the current invention to provide a system for detecting defects on a wafer, the system having the ability to detect defects beyond those detectable using monochromatic or narrowband brightfield imaging alone. Inherent in such a system would be the ability to minimize contrast variations and coherent noise problems.
It is another object of the current invention to provide a system for detecting defects which has the ability to detect small particles, including particles having a size smaller than the wavelength of light, with a minimum number of false detections. The system should provide for a minimum number of components to decrease overall cost and provide for maximum throughput of specimens.
It is yet another object of the current invention to provide a system which provides the ability to perform an accurate inspection of Chemical Mechanical Planarized (CMP) wafers, or other specimens having film thickness variations or grainy textures.
It is still another object of the current invention to provide a system for detecting defects on a wafer wherein the system has the ability to optimize the incidence of light reflected from specimens having various surface characteristics.
It is still another object of the current system to provide both brightfield and darkfield illumination using a minimum number of components in a minimum amount of physical space while simultaneously minimizing adverse effects associated with brightfield and darkfield illumination.
It is still another object of the current invention to provide an efficient method or apparatus for light level control in the wafer inspection system.